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Pharmacokinetics of Nelarabine and 9-Beta-D-Arabinofuranosyl Guanine in Pediatric and Adult Patients During a Phase I Study of Nelarabine for the Treatment of Refractory Hematologic Malignancies

From the Ohio Northern University, Ada, OH; M.D. Anderson Cancer Center, Houston, TX; Duke University, Durham, University of North Carolina, Chapel Hill, and Glaxo Wellcome Inc, Research Triangle Park, NC; and Boston University Medical Center, Boston, MA.

Address reprint requests to David F. Kisor, Ohio Northern University, College of Pharmacy, Ada, OH 45810; email d-kisor{at}onu.edu

PURPOSE: To characterize the pharmacokinetics of nelarabine (506U78), the water-soluble prodrug of 9-beta-D-arabinofuranosyl guanine (ara-G), and ara-G in pediatric and adult patients with refractory hematologic malignancies. Ara-G is phosphorylated within leukemic cells to form ara-G triphosphate (ara-GTP), which acts to terminate DNA chain elongation, resulting in cell death.

PATIENTS AND METHODS: The pharmacokinetics of nelarabine and/or ara-G were evaluated in 71 patients (25 pediatric and 46 adult patients) on the first day of therapy. Blood was collected at specified times for the determination of plasma nelarabine and ara-G concentrations.

RESULTS: There were no statistically significant differences in the pharmacokinetics of nelarabine between any of the groups of patients. The harmonic mean half-life (t1/2) of nelarabine in pediatric and adult patients was 14.1 minutes and 16.5 minutes, respectively. The maximum concentrations (C_max) of ara-G occurred at or near the end of the nelarabine infusion. The C_max of ara-G ranged from 11.6 µmol/L to 308.7 µmol/L at nelarabine doses of 5 to 75 mg/kg and was linearly related to the nelarabine dose. No statistically significant differences were noted for the pharmacokinetic parameter estimates of ara-G between adult male and female patients. In children versus adults, the dose-normalized C_max, time of the C_max, and the steady-state volume of distribution of ara-G were similar. However, the clearance of ara-G was higher in pediatric patients (0.312 L·h^-1·kg^-1) as compared with adult patients (0.213 L·h^-1·kg^-1) (P < .001). The t1/2 of ara-G was shorter in pediatric patients as compared with adult patients (2.1 hours v 3.0 hours; P < .01).

CONCLUSION: Nelarabine is an effective prodrug of ara-G, allowing systemic concentrations of ara-G that result in clinical activity.

This work is dedicated to the memory of Gertrude B. Elion, mentor, colleague, and friend.

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